Sensory organs and structures in animals

Channels for information to be delivered to the brain (“biological transducers”)

What types of stimuli? (what forms of energy?)electricalmechanicalchemicalradiant

Stimuli are transformed into nerve impulsesinterpreted by specialized areas of the brain(Muller, 1830s)

What are receptors?specialized neurons or epithelial cellsoften located in special sensory organs

Exteroreceptors- detect external environment;located on or near body surface

Interoreceptors detect stimuli within the body

Proprioceptors- muscle tension; body position

Energy is converted to membrane potentials

Sensory transduction (e.g., taste)G protein pathway

Taste involves chemoreceptors (for chemicalsin solution)

So does smell (for airborne chemicals)

A stimulus may be amplified

Accessory structures in sensory organ

Some receptors are more sensitive than others

Touch receptors (along with hearing andequilibrium) utilize mechanoreceptors

Pain receptors (nocireceptors) harder tostimulate, thankfully

Types of pain receptorsproprioreceptors- body position and

movement

cutaneous receptors (aka generalized)touch, pressure, pain, heat, cold

special senses- sight, hearing, taste, smell

Sensory adaptation

Phasic receptors- slow down after initial stimulusadaptation- e.g., odor, touch, temperaturefast-adapting

Tonic receptors- fire at constant rateslow-adapting

Receptors deliver specific stimulus; brain filtersout a lot of the stimuli

Produce action potential in response to stimulus

Cutaneous sensations

Different neurons perceive different sensationsFree nerve endings- light touch, temperature, pain

Merkel’s discs-sustained touch and pressure(superficial)

Ruffini’s corpuscles sustained pressure (deep)

Meissner’s corpuscles- texture, slow vibrationPacinian corpuscles- deep pressure, fast

vibration

Cold receptors are more numerous and closerto surface than heat receptors

Cold receptors are inhibited by warming

Sharp pain- conducted by myelinated axonsdull pain by unmyelinated axons

Capsaicin receptor- ion channel; producessensation of pain in response to high heat

Neural pathways to postcentral gyrus

Ascending fibers in dorsal columns of spinal cord

Synapse in medulla oblongata

Ascends to thalamus

Sensory neurons project to postcentral gyrus

Receptive fields in skinlarge if receptors are few, small if receptorsare dense

measured by two-point touch threshold

Lateral inhibition- sensory neurons most stronglyaffected by a stimulus inhibit others innearby receptive fields

Hair cells are mechanoreceptors

Often project into a fluid-filled compartment (asin inner ear)

Sound receptors systems in animals

Arthropods- air pockets surrounded by tympanic membrane (most invertebrates have no“hearing”)

Some moths can detect ultrasonic waves (to avoid bats that prey on them

Vertebrate ear is much more complex!Human ear is example, of course

Evolution of middle ear

Amphibians, reptiles and birds have a singleossicle (columella; stapes)

Stapes originated in fish, as a jaw support

Other ossicles also developed from earlyjaw structures

Equilibrium in invertebrates

Statocysts function like utricle and saccule

Statoliths “acquired” from environment

Lateral line

Fishes and (aquatic) amphibians

Analogous to inner ear

Photoreception (vision)

Some invertebrates have light-sensitive cellsscattered over their bodieslocomotion, reproductive behaviors

The dinoflagellate Nematodinium has a lens,light-gathering chamber and photoreceptivepigments- and is single-celled

Cannot form image

Invertebrates: single-lens or compound eyes

Compound: insects, crustaceans, some annelids

Cannot focus image well but can see movement

Insects can see color, and some can see inultraviolet range

Single-lens eyes

Annelids, mollusks, spiders, vertebrates

Types of photoreceptors

Rods- light intensity

Cones- color vision, visual acuity

Many more rods than cones

In human, only cones are found on thefovea centralis

Rods can make more rhodopsin in the dark

Brain actually determines what we see